Cardiac Output Measurement
DEFINITIONS
Q = SV x HR (L/min)
CI = SV x HR/BSA (L/min/m2)
Normal CI = 2.5-4.2
SV = end-diastolic volume – end systolic volume
EF = (SV/EDV) x 100%
Shock = failure of tissue perfusion -> end organ injury
THE FICK PRINCIPLE
- Adolf Eugen Fick (1829-1901) in 1870, was the first to measure cardiac output
- assumes oxygen consumption is a function of rate of blood flow and rate of oxygen pick pick up by RBC’s.
- involves measurement of oxygen concentration of arterial and venous blood and subsequent calculation of O2 consumption.
- Q can then be derived
Measurements
- VO2 = oxygen consumption/min (from spirometer with subject rebreathing air through a CO2 absorber)
- Cv = oxygen content of blood taken from pulmonary artery (deoxygenated)
- Ca = oxygen content of blood taken from a peripheral artery (oxygenated)
Calculations
VO2 = (Q x Ca) – (Q x Cv)
Therefore,
Q = VO2/(Ca-Cv)
Issues:
- impractical
- assumes no shunt (pulmonary blood flow = systemic blood flow)
- assumes arterial blood is equal to pulmonary venous blood
DILUTION TECHNIQUES
- known quantity of tracer substance introduced into a space to be measured
- concentration measured after complete mixing
C1 x V1 = C2 x (V1 + V2)
C1 = initial concentration of indicator
C2 = final concentration of indicator
V1 = volume of indicator
V2 = volume to be measured
- marker injected proximally to right ventricle and concentration measure distally (pulmonary artery or a peripheral artery)
- concentration vs time plotted -> integration allow calculation of area under curve (SV x HR = Q)
- suitable substances: radioiosotope, dye, cold water, temperature of blood.
TECHNIQUES
Clinically Used
- Non-invasive BP monitoring
- Central venous monitoring
- Arterial monitoring
- Pulmonary arterial monitoring
- ECHO: TOE and TTE
- Pulse contour analysis (PiCCO)
- Oesophageal Doppler
- Cardiac catheterisation and angiography
Experimental
- Aortovelography – dopper U/S probe in suprasternal notch to measure blood velocity and acceleration in ascending aorta.
- Ballistocardiography – detection of body motion due to movement of blood within body with each heart beat.
- Electromagnetic flow meters
- Oxygen consumption estimation (Fick)
- Impedance plethymography
TIPS WHEN USING CARDIAC OUTPUT MONITORS
- there is no ‘normal’ CVP or wedge -> follow trend and look at the response to treatment
- abnormal hearts (ischaemic, fibrotic, contused) are less compliant so require higher filling pressures to reach ‘normal’ SV.
- use SV rather than Q as a response to treatment as Q is calculated from HR which may be fast and mask a poorly performing ventricle.
- low SvO2 usually indicates under-resuscitation.
- the first treatment for all shock (including cardiogenic) = volume, volume and more volume.
- a little extravascular lung water is less harmful than vasoactive drugs.
- there is no formula to calculate the effect of PEEP on PCWP and CVP -> if kept constant, trend should be consistent.
- during resuscitation if becomes apparent what CVP the patient ‘likes’ -> aim for this.
- be cautious of all derived variables, particularly SVR.
References and Links
Introduction to ICU Series
Introduction to ICU Series Landing Page
DAY TO DAY ICU: FASTHUG, ICU Ward Round, Clinical Examination, Communication in a Crisis, Documenting the ward round in ICU, Human Factors
AIRWAY: Bag Valve Mask Ventilation, Oropharyngeal Airway, Nasopharyngeal Airway, Endotracheal Tube (ETT), Tracheostomy Tubes
BREATHING: Positive End Expiratory Pressure (PEEP), High Flow Nasal Prongs (HFNP), Intubation and Mechanical Ventilation, Mechanical Ventilation Overview, Non-invasive Ventilation (NIV)
CIRCULATION: Arrhythmias, Atrial Fibrillation, ICU after Cardiac Surgery, Pacing Modes, ECMO, Shock
CNS: Brain Death, Delirium in the ICU, Examination of the Unconscious Patient, External-ventricular Drain (EVD), Sedation in the ICU
GASTROINTESTINAL: Enteral Nutrition vs Parenteral Nutrition, Intolerance to EN, Prokinetics, Stress Ulcer Prophylaxis (SUP), Ileus
GENITOURINARY: Acute Kidney Injury (AKI), CRRT Indications
HAEMATOLOGICAL: Anaemia, Blood Products, Massive Transfusion Protocol (MTP)
INFECTIOUS DISEASE: Antimicrobial Stewardship, Antimicrobial Quick Reference, Central Line Associated Bacterial Infection (CLABSI), Handwashing in ICU, Neutropenic Sepsis, Nosocomial Infections, Sepsis Overview
SPECIAL GROUPS IN ICU: Early Management of the Critically Ill Child, Paediatric Formulas, Paediatric Vital Signs, Pregnancy and ICU, Obesity, Elderly
FLUIDS AND ELECTROLYTES: Albumin vs 0.9% Saline, Assessing Fluid Status, Electrolyte Abnormalities, Hypertonic Saline
PHARMACOLOGY: Drug Infusion Doses, Summary of Vasopressors, Prokinetics, Steroid Conversion, GI Drug Absorption in Critical Illness
PROCEDURES: Arterial line, CVC, Intercostal Catheter (ICC), Intraosseous Needle, Underwater seal drain, Naso- and Orogastric Tubes (NGT/OGT), Rapid Infusion Catheter (RIC)
INVESTIGATIONS: ABG Interpretation, Echo in ICU, CXR in ICU, Routine daily CXR, FBC, TEG/ROTEM, US in Critical Care
ICU MONITORING: NIBP vs Arterial line, Arterial Line Pressure Transduction, Cardiac Output, Central Venous Pressure (CVP), CO2 / Capnography, Pulmonary Artery Catheter (PAC / Swan-Ganz), Pulse Oximeter
Critical Care
Compendium
